Not Applicable.
1. Field of Invention
The present invention relates to an apparatus and method for the measurement of inductance. More precisely the present invention relates to an apparatus and method for the measurement of inductance of vehicles moving in a traffic lane using permeability-modulated carrier referencing.
2. Description of the Related Art
It is well known in the prior art to measure the inductance of a wire-loop, which is part of the frequency determining circuit of an inductance-capacitance-resistance (xe2x80x9cLCRxe2x80x9d) oscillator, using frequency-counting techniques. Typically, the number of zero-crossings per time increment of the voltage across the terminals of the LCR capacitor is counted. Because the frequency of the LCR oscillator is inversely proportional to the square root of the inductance of the LCR circuit, changes in the inductance of the wire-loop are reflected in changes of the number of zero-crossing counted per time increment.
The Class-C wire-loop oscillator described in U.S. Pat. No. 3,873,964 issued to Thomas R. Potter on Mar. 25, 1975 is typical of LCR oscillators used in the prior-art. When a vehicle passes over a wire-loop connected to a running LCR oscillator, the metal of the vehicle changes the permeability of some of the space surrounding the wire-loop causing modulation of the carrier wave generated by the LCR oscillator. Changes in the inductance of the wire-loop caused by the vehicle are thus superimposed onto the LCR oscillator""s carrier wave, yielding a permeability-modulated carrier. Next, the inductive signature is retrieved from the permeability-modulated carrier. One method of demodulating the carrier is the use of frequency counting techniques, such as with xe2x80x9csignature cardsxe2x80x9d which are commercially available from 3M Corporation and Peek Traffic. The signature cards offer approximately a 100 Hz-sample rate, which is not fully adequate for demodulating the inductive signatures of vehicles moving at highway speeds.
Another problem associated with the measurement of inductance in a wire-loop is crosstalk. Crosstalk between two or more wire-loops is a result of inductive coupling between the wire-loops, which results in energy transfer between the wire-loops when a changing current is flowing through them. If two wire-loops are operating at nearly the same frequency, then the energy transfer can result in an exaggerated buildup, or stagnation, of transferred energy in one LCR circuit, and a corresponding exaggerated energy depression in the other. This can cause the carrier waves of the two circuits to become entrained with each other in a more-or-less fixed phase differential and effectively eliminates the ability of the wire-loops to detect vehicles independently of each other. Typically, an inductive coupling coefficient of only a few percent is sufficient to cause complete entrainment. In prior-art vehicle detectors, carrier wave entrainment due to crosstalk is partially avoided by operating the oscillator circuits associated with the wire-loops at different frequencies, typically by varying the value of the capacitance, C, of the LCR circuit. This can prevent stagnation and entrainment, but does not address the underlying errors induced into each detector by the energy transfer due to mutual inductive coupling.
Accordingly, there is a need for an apparatus and method for measuring the changes in the inductance of a wire-loop caused by a vehicle traveling along a monitored roadway. The apparatus and method need be capable of measuring changes in the inductance of a wire-loop caused by a vehicle traveling at highway speeds. Further, the apparatus and method should be capable of measuring inductance without attempting to identify frequency changes. Finally, there is a need for an apparatus and method capable of measuring inductance using multiple inductive sensors without significant errors resulting from crosstalk.
Therefore, it is an object of the present invention to provide an apparatus and method for measuring changes in the inductance of a wire-loop caused by a vehicle traveling along a monitored roadway. is another object of the present invention to provide an apparatus and method for measuring changes in the inductance of a wire-loop caused by a vehicle traveling at highway speeds.
It is a further object of the present invention to provide an apparatus and method for measuring changes in the inductance of a wire-loop caused by a vehicle and producing an inductive signature for that vehicle.
It is yet another object of the present invention to provide an apparatus and method for measuring changes in the inductance of a wire-loop caused by a vehicle and producing an inductive signature of that vehicle by referencing a measured voltage to a permeability-modulated current carrier wave.
A still further object of the present invention is to provide an apparatus and method for measuring changes in the inductance of a wire-loop caused by a vehicle using multiple inductive sensors without significant errors resulting from crosstalk.
Another object is to provide an apparatus and method for measuring changes in the inductance of a wire-loop caused by a vehicle which does not need to be installed in the driving surface of a roadway.
In a typical LCR circuit, a number of factors are related to the value of the inductance. For example, the frequency is inversely proportional to the square root of the inductance, L. This relationship is a consequence of the direct dependence of the instantaneous rate of change in current flow, xcex4I, upon the value of the inductance. Accordingly, frequency is only an indirect indication of this more general relationship because the circuit voltage, V, is in turn a function of current, I, and capacitance, C. A more direct indication of inductance in an LCR oscillator is the amplitude of the current function, I(t), which is inversely proportional to the inductance of the LCR circuit. The changing current function, I(t), in the LCR circuit of an inductive vehicle detector is a permeability-modulated carrier. This carrier is modulated in both frequency and amplitude by the changing apparent permeability of the space surrounding a wire-loop caused by the motion of a nearby metallic object, typically an automotive vehicle. It should be noted that induced electromagnetic noise, such as from high voltage power lines, also effectively modulates the current function carrier wave. However, the induced noise modulates the voltage function, V(t), in an asymmetric manner by shifting the voltage function on the magnitude axis. Because the modulation resulting from the induced noise affects the current flow and the voltage function differently, the permeability-modulated current carrier function, I(t), can be cross-referenced with the voltage function, V(t), to isolate the desired inductance from the induced noise. This method of isolating the inductance is known as permeability-modulated carrier referencing (PMCR). PMCR is particularly effective at removing low-frequency induced noise from an inductance measuring circuit. Those skilled in the art will recognize that although PMCR is described herein with reference to an LCR oscillator, the principles are equally applicable to other forms of carrier functions including, but not limited to, pulsed-type discrete cycle inductance measurement techniques.
Another factor affecting the performance of the present invention is crosstalk wherein the direction of current flow in an inductor determines the direction of the induced differential current flow in inductors that are inductively coupled to it. One method of reducing crosstalk is to nullify the underlying mutual inductive coupling of a plurality of wire-loops using passive transformers. The passive transformer inductively couples the inductors in precisely the opposite polarity and magnitude in which they were originally coupled nullifying the original coupling and eliminating the potential for crosstalk at the source. In addition to removing the gross errors introduced by crosstalk, nullification of the inductive coupling also removes the more subtle transient errors in the detectors, which appear as non-repeatable errors in recorded inductive signatures.
A single-turn wire-loop spanning the width of one or more traffic lanes is sufficient to detect the speed, the direction, the lane position, and the wheelbase dimensions for any vehicle passing over the wire-loop. The speed and the lateral lane position of a vehicle are unambiguously determined if the two active legs of the wire-loop span the traffic lanes at different skew angles. Symmetric skew angles also produce useful data, but are ambiguous in resolving the vehicle direction. Similar skew angles are unable to resolve the lane position; however, this is not as important for single traffic lanes as it is for multiple traffic lanes. Finally, zero skew angles can produce speed and axle-count data, but are ambiguous in resolving vehicle direction, can not resolve the lane position or the width of the wheelbase, and are ambiguous with respect to vehicle continuity when multiple traffic lanes are involved.
For multi-lane traffic, a pair of single-turn wire-loops in a complimentary wedge-shaped configuration is ideal for collecting the maximum unambiguous traffic-flow data. This configuration is a hybrid of rectangular wire-loops and blades which gives repeatability of signatures that is characteristic of the blades along with the less-intrusive installation that is characteristic of simple wire-loops. Shallow saw-cuts are desirable for a traffic sensor spanning long distances in a pavement surface to prevent the formation of a shear-plane and slot faulting.
A large-aperture wire-loop can detect metallic objects at great distances. The magnetic field generated by a wire-loop is highly directional at a significant distance from the wire-loop. More precisely, a wire-loop is most sensitive to distant objects in the same plane as the wire-loop and sensitivity decreases as the object moves away from the plane of the wire-loop. At a significant distance, objects approaching the plane perpendicular to that of the wire-loop are virtually invisible to the wire-loop. This directional sensitivity of the wire-loop is useful in determining the relative direction to detected objects in a similar way as a radar antenna is directional. Large-aperture wire-loops are used in non-intrusive vehicle-detecting applications because they do not need to be embedded in or laid on the pavement to detect passing vehicles.